Feedback in hearing aids is often compensated for by adaptive filters. In doing so, the filter is appropriately adapted to suit the feedback situation. The adaptation is carried out in steps. The problem here is that in many situations a rapid adaptation of the filter is desired and in other situations a rather slower adaptation of the filter is desired. Accordingly, a suitable step size must be found for the adaptation.
Previously, this problem has been solved, for example, by shadow filter methods, methods for amplitude or phase modulation of the output signals and so on. However, most of these approaches have considerable weaknesses, in particular when it comes to differentiating strongly correlated excitation signals such as music, the clinking of glass or cutlery, for example, from feedback(s) or feedback signal(s). With strongly correlated excitation signals, the step size should namely be reduced in order to prevent an incorrect adaptation of the filter and therefore signal distortion due to resulting feedback. With less correlated signals, e.g. in the case of white noise, the step size should be rapidly increased in order to terminate the feedback resulting from a changed acoustic environment as quickly as possible, e.g. a hand in the vicinity of the hearing aid.
A method for controlling a hearing aid for adjusting the adaptation in situ is disclosed in patent specification EP 0 824 845 B1. If noticeable feedback is ascertained when operating the hearing aid, the maximum gain is reduced for at least one of several frequency bands. The gain in all other frequency bands remains unchanged so that the transmission function is to be adapted for at least one frequency band.
Furthermore, a method for reducing feedback in an acoustic system is described in patent specification DE 10 2004 050 304 B3. Here, an output signal and therefore also a feedback signal is modulated so that the feedback signal can be detected. The information relating to the presence of feedback can be used for controlling the step size of an adaptive compensation filter. A filter of this kind can be used for partial bands and also for the complete band.